It is well known that there is a steadily increasing demand for higher performance materials in optical applications. In many cases, these materials must be high quality single crystals of a size sufficiently large so that they are capable of being cut, shaped and polished into pieces several millimeters on a side. This is particularly true for solid state optical devices such as all solid state lasers and optical switching devices. For example, there has recently been a rapidly expanding application of new crystals finding use in diode pumped solid state lasers.
More specifically, there is a rapidly increasing demand for lasers capable of generating coherent radiation in the violet and ultraviolet region of the optical spectrum. In general these shorter wavelengths of coherent radiation have many useful properties. Shorter wavelength leads to greater resolution in applications such as lithography, micromachining, patterning, labeling, information storage and related applications. In addition a convenient source of UV radiation would lead to significant advances in spectroscopy, biological applications and sensor technology. At present there are very few methods available for the generation of coherent laser radiation at wavelengths between 150 and 350 nm. The most common techniques rely on excimer lasers based on gases like krypton fluoride or fluorine, capable of generating 193 nm and 157 nm laser radiation respectively. However, these lasers require the use of corrosive gases. As such they are large, bulky, unreliable and restricted to a few specific wavelengths. Diode lasers that emit in the UV are the subject of intense research and, although showing some promise, are plagued by short lifetimes, low power and generally limited performance.
An attractive and simple alternative for UV lasers is the generation of short wavelength laser radiation by multiple harmonic generation of readily available longer wavelength laser sources using non-linear optical frequency multiplying crystals. Such an approach is typified in the visible region by the generation of 532 nm coherent radiation by the second harmonic generation of 1064 nm emission generated by conventional Nd:YAG, Nd:YVO4 or related sources. In the case of 532 nm radiation, the non-linear optical crystal used for the frequency doubling is most typically K(TiO)(PO4) (KTP). The process generally can be used to generate relatively high powers, and employs solid crystals making the devices very reliable, compact and long lasting.
Frequency doubling is a non-linear optical process that combines two photons of one wavelength to produce a new photon of one half the wavelength. Thus it is energy neutral. The process is not notably efficient but requires only passive optical components. The checklist of requirements for a successful second harmonic generation crystal is well understood. The crystal must grow in a space group having no center of symmetry and be a member of an acentric point group in a uniaxial or biaxial lattice type (rhombohedral, hexagonal, tetragonal, trigonal, orthorhombic or monoclinic) with a satisfactory non-linear optical (NLO) coefficient. In addition, the crystal must have moderate birefringence for phase matching, and must be transparent and optically stable to both the excitation and generated wavelengths. The crystal must have good physical properties, be thermally stable, hard enough to fabricate and polish and be mechanically stable and non-hygroscopic.
Most importantly, the crystal must be able to be produced in high quality in a suitable size (typically several millimeters per side or larger) for optoelectronic applications. For generation of light in the visible region and the near IR, several excellent crystals exist that fulfill these requirements, including KTP, KTA, LiNbO3 and KNbO3. However, none of these are suitable for generation of near UV, UV or deep UV radiation as their band gaps are not large enough, so they absorb all of the resultant second harmonic radiation. In the past two decades, several new crystals have been introduced to fulfill the minimum requirements described above. Borates are especially attractive because they often have band gaps that are sufficiently wide to accommodate UV radiation and tend to crystallize in acentric space groups. The most common of these is β-BaB2O4 (BBO) that is typically used for the generation of 266 nm radiation through the frequency doubling of 532 nm radiation. Several other crystals with unique structures have also been introduced recently including LiB3O5 (LBO) and CsLiB6O10 (CLBO). All these materials have some commercial availability.
Although these materials do crystallize in acentric space groups and have sufficiently wide absorption band gaps to allow formation of radiation in part of the UV spectrum, they all have considerable limitations. For example BBO has an insufficiently wide band gap to allow generation of radiation with wavelengths shorter than 220 nm. In particular the presence of a B3O6 ring in the crystal lattice leads to the presence of a band gap that is not large enough for frequency mixing below 200 nm. The other crystals currently used for non-linear optical applications in the UV are LBO and CLBO. Both of these also have severe limitations. In particular, LBO has an extremely low NLO coefficient and does not phase-match well, while CLBO is very hygroscopic. There are numerous other crystals, typically borates that have been reported as growing in acentric space groups and having suitable wide band gaps for UV radiation. However, they almost all have insufficient birefringence to allow for phase matching. The inability to phase-match is an important limitation that renders these otherwise promising crystals useless for any practical NLO related applications. Thus there is considerable demand for new crystals that can meet the criteria for frequency multiplication over a reasonably wide range of the UV spectrum.
In the last decade two new berylloborates with the formulas KBe2BO3F2 (KBBF) and Sr2Be2B2O7 (SBBO), were reported that seem to fulfill most of the necessary requirements. They have band gaps near 155 nm and grow in appropriate acentric space groups. Most importantly they seem to have moderate birefringence and appear capable of phase matching over a fairly wide range below 220 nm. Thus they seem like very promising candidates for generation of a wide range of UV and deep UV laser radiation using well-understood solid-state technology. Preliminary reports suggest that both materials KBBF and SBBO have properties that make them suitable for numerous applications requiring non-linear optical behavior.
However, there has been a serious limitation to the practical introduction of the SBBO in any of the above applications. All of the applications described herein require high quality single crystals of a certain size, typically 3-5 millimeters after cutting and polishing. To be of any use the materials must be high quality single crystals of a size sufficiently large so that they are capable of being cut, shaped and polished into pieces several millimeters on a side. Thus any useful application requires raw product in the form of single crystals at least one centimeter per edge. For commercial production, the size must be at least several centimeters per edge. These same formulas in the form of microcrystals, powders or small, low quality crystals are essentially worthless for optoelectronic applications. Only large, high quality crystals provide material with any technical use. This is particularly true for solid state optical devices such as all solid state lasers and optical switching devices.
However, the growth of SBBO is extremely problematic. Although it has been reported to grow out of molten fluxes, the growth method has not proven to be reproducible or suitable for crystals of sufficient quality for any optical application. This dramatic shortcoming has prevented the introduction of this promising material into any prototype device and has even limited the further measurement of the physical properties of the materials. Despite repeated attempts by the original authors they report that they have been unable to produce satisfactory single crystals using any flux technology. The products are always too small, cracked, flaky and generally of too poor quality for any conceivable optical application.
Hydrothermal techniques are an excellent route to high quality single crystals for electro-optic applications. For example, all electronic grade quartz is grown commercially by the hydrothermal method. Further, KTP is grown by both flux and hydrothermal methods, and it is widely acknowledged by those skilled in the art that the hydrothermally grown product is of generally superior quality. The hydrothermal method involves the use of superheated water (liquid water heated above its boiling point) under pressure to cause transport of soluble species from a nutrient rich zone to a supersaturated growth zone. Generally a seed crystal is placed in the growth zone. The growth and supersaturation control is achieved by the use of differential temperature gradients. The superheated fluid is generally contained under pressure, typically 5-30 kpsi, in a metal autoclave. Depending on the chemical demands of the system the autoclave can be lined with a nobel metal using a either fixed or floating liner. These general techniques are well known in the art and have been used for the growth of a variety of other electro-optic crystals.